The present invention relates in general to wireless telecommunications systems and more particularly to techniques for processing data transmitted and received over a wireless link connecting a central terminal and a subscriber terminal of a wireless telecommunications system.
A wireless telecommunications system has been proposed in which a geographical area is divided in to cells, each cell having one or more central terminals (CTs) for communicating over wireless links with a number of subscriber terminals (STs) in the cell. These wireless links are established over predetermined frequency channels, a frequency channel typically consisting of one frequency for uplink signals from a subscriber terminal to the central terminal, and another frequency for downlink signals from the central terminal to the subscriber terminal.
Due to bandwidth constraints, it is not practical for each individual subscriber terminal to have its own dedicated frequency channel for communicating with the central terminal. Hence, techniques need to be applied to enable data items relating to different wireless links to be passed over the same frequency channel without interfering with each other. In current wireless telecommunications systems, this can be achieved through the use of xe2x80x98Code Division Multiple Accessxe2x80x99 (CDMA) technique. One way to implement CDMA is through the application of a set of orthogonal codes to the data items to be transmitted on a particular frequency channel, data items relating to different wireless links being combined with different orthogonal codes from the set. A suitable set of orthogonal codes is a xe2x80x9cRademacher-Walshxe2x80x9d (RW) set of sixteen 16-bit codes. Orthogonal codes have the property that, when perfectly aligned, all codes cross-correlate to zero, thus making it possible to decode a signal to which one orthogonal code has been applied while cancelling interference from signals to which different orthogonal codes have been applied.
Signals to which an orthogonal code has been applied can be considered as being transmitted over a corresponding orthogonal channel within a particular frequency channel. Hence, considering-the example of a set of 16 RW codes, 16 orthogonal channels can be created within a single frequency channel, and hence up to sixteen separate communication signals (corresponding to sixteen separate wireless links) can be transmitted simultaneously over the single frequency channel if different RW codes are applied to each communication signal.
It is known to provide a number of modem shelves within one central terminal, and for each modem shelf to employ a different frequency channel. Hence, if a central terminal has four modem shelves, and the set of 16 RW codes is employed for each frequency channel, one central terminal would be able to support wireless links with up to 60 subscriber terminals simultaneously.
However, as more subscribers subscribe to the wireless telecommunications network, it is becoming desirable to support more and more subscriber terminals from each central terminal. There are only a limited number of frequency channels that can be allocated to the wireless telecommunications system, and as it is desirable for neighbouring cells to use different frequency channels so as to reduce interference, the demand cannot be met by merely adding more modem shelves to each central terminal.
According to the present invention, there is provided a transmission controller for processing data items to be transmitted over a wireless link connecting a central terminal and a subscriber terminal of a wireless telecommunications system, a single frequency channel being employed for transmitting data items pertaining to a plurality of wireless links, the transmission controller comprising: an orthogonal code generator for providing an orthogonal code from a set of xe2x80x98mxe2x80x99 orthogonal codes used to create xe2x80x98mxe2x80x99 orthogonal channels within the single frequency channel; a first encoder for combining a data item to be transmitted on the single frequency channel with said orthogonal code from the orthogonal code generator, the orthogonal code determining the orthogonal channel over which the data item is transmitted, whereby data items pertaining to different wireless links may be transmitted simultaneously within different orthogonal channels of said single frequency channel; and a TDM encoder arranged to apply time division multiplexing (TDM) techniques to the data item in order to insert the data item within a time slot of the orthogonal channel, whereby a plurality of data items relating to different wireless links may be transmitted within the same orthogonal channel during a predetermined frame period.
Viewed from a second aspect, the present invention provides a reception controller for processing data items received over a wireless link connecting a central terminal and a subscriber terminal of a wireless telecommunications system, a single frequency channel being employed for transmitting data items pertaining to a plurality of wireless links, and xe2x80x98mxe2x80x99 orthogonal channels being provided within the single frequency channel, the receiver controller comprising: an orthogonal code generator for providing an orthogonal code from a set of xe2x80x98mxe2x80x99 orthogonal codes used to create said xe2x80x98mxe2x80x99 orthogonal channels within the single frequency channel; a first decoder for applying, to signals received on the single frequency channel, the orthogonal code provided by the orthogonal code generator, in order to isolate data items transmitted within the corresponding orthogonal channel; and a TDM decoder arranged to extract a data item from a predetermined time slot within said orthogonal channel, a plurality of data items relating to different wireless links being transmitted within the same orthogonal channel during a predetermined frame period.
By using TDM techniques in addition to the known set of orthogonal codes, it is possible for selected orthogonal channels to be subdivided in the time dimension. For example, if TDM is used to divide one frame period in to four subframes, and each orthogonal channel is subject to the TDM technique, then up to 64 separate communication signals can be transmitted on the sixteen orthogonal channels during one frame period, albeit at a quarter of the rate that the communication signals could be transmitted if the TDM technique was not used.
Such an approach has the advantage that it preserves compatibility with current hardware and software equipment which use the set of orthogonal codes, but which do not support the use of TDM techniques. By designating certain orthogonal channels as channels for which TDM is not used, the current equipment can communicate over those channels without any changes being required to the equipment.
In preferred embodiments, the transmission controller further comprises: an overlay code generator for providing an overlay code from a first set of xe2x80x98nxe2x80x99 overlay codes which are orthogonal to each other; and a second encoder, selectively operable instead of the TDM encoder, to apply the overlay code from the overlay code generator to said data item, whereby xe2x80x98nxe2x80x99 data items pertaining to different wireless links may be transmitted simultaneously within the same orthogonal channel.
Similarly, the reception controller may further comprise: an overlay code generator for providing an overlay code from a first set of xe2x80x98nxe2x80x99 overlay codes which are orthogonal to each other, the set of xe2x80x98nxe2x80x99 overlay codes enabling xe2x80x98nxe2x80x99 data items pertaining to different wireless links to be transmitted simultaneously within the same orthogonal channel; and a second decoder, selectively operable instead of the TDM decoder, to apply to the data items of the orthogonal channel, the overlay code from the overlay code generator so as to isolate a particular data item transmitted using that overlay code.
By such an approach, data items transmitted within certain orthogonal channels can be encoded using TDM techniques whilst data items transmitted within other orthogonal channels can be encoded using overlay codes, the reception controllers including the necessary decoders to decode either type of encoded data item. A preferred arrangement, where certain orthogonal channels are subject to TDM techniques whilst others are subject to overlay codes, will be discussed in more detail later.
The orthogonal code generator may be arranged to generate orthogonal codes xe2x80x98on the flyxe2x80x99 using predetermined algorithms. However, alternatively, the orthogonal code generator may be provided as a storage arranged to store the set of orthogonal codes. Appropriate orthogonal codes can then be read out to the encoder or decoder from the storage as required.
In preferred embodiments, the set of orthogonal codes comprise a set of Rademacher-Walsh (RW) codes, in preferred embodiments the set comprising a 16xc3x9716 matrix of RW codes.
The transmission controller in accordance with the present invention may be provided within the central terminal of a wireless telecommunications system. In preferred embodiments, the central terminal would further comprise channelisation means for determining which of the orthogonal channels will be subject to TDM techniques, and for transmitting that information to a plurality of subscriber terminals within the wireless telecommunications system. This is useful since, for example, certain orthogonal channels can hence be designated as being reserved for communications with STs that do not incorporate the features necessary to support TDM techniques, and which hence require the full orthogonal channel for the whole frame period.
In preferred embodiments, the channelisation means also determines, for those orthogonal channels subject to TDM techniques, how many time slots will be provided within each orthogonal channel. This gives a great deal of flexibility in how channels are used, since some can be subdivided in the time dimension whilst others are not, and those which are subdivided can be subdivided differently to yield differing numbers of time slots per frame period. For instance, if an orthogonal channel operates at 160 kb/s, and four time slots are provided within that orthogonal channel in order to carry data items pertaining to four different wireless links during one frame period, then each ST receiving data from said orthogonal channel will receive data at a rate of 40 kb/s (since each ST will only read a quarter of the data transmitted on the orthogonal channel during each frame period). If, alternatively, two time slots are provided within the orthogonal channel, then data items pertaining to only two different wireless links will be transmitted per frame period, and the two STs receiving data will do so at a rate of 80 kb/s (since each ST will read half of the data transmitted on the orthogonal channel during one frame period). This flexibility is useful, since for some communications, eg. fax, a rate of 40 kb/s may not be acceptable, and hence the use of four time slots would not be suitable.
In preferred embodiments, a number of said orthogonal channels are designated as traffic channels for the transmission of data items relating to communication content, and the TDM encoder is employed to apply time division multiplexing (TDM) techniques to data items to be sent over a traffic channel from said central terminal to said subscriber terminal. The use of this CDMA/TDM hybrid approach for downlink traffic channels retains the benefit of CDMA access, ie. interference is reduced when traffic is reduced, and also reduces receiver dynamic range requirements.
However, a first of the orthogonal channels is preferably reserved for the transmission of signals relating to the acquisition of wireless links, and the second encoder is used instead of the TDM encoder to enable overlay codes to be applied to data items to be sent within said first orthogonal channel from the central terminal to one of said subscriber terminals. Similarly, a second of the orthogonal channels is preferably reserved for the transmission of signals relating to the control of calls, and the second encoder is used instead of the TDM encoder to enable overlay codes to be applied to data items to be sent within said second orthogonal channel from the central terminal to one of said subscriber terminals.
In preferred embodiments, at least one of the subscriber terminals of a wireless telecommunications system comprises a reception controller in accordance with the present invention. However, for transmission of data from subscriber terminals, it is preferable for the ST to have a transmission controller which employs overlay codes for all types of orthogonal channels, whether they be traffic channels or otherwise. On these uplink channels, the pure CDMA approach using overlay codes eliminates the need to time synchronise STs to a TDM frame reference, and reduces the peak power handling requirements in the ST RF transmit chain.
Viewed from a third aspect, the present invention provides a wireless telecommunications system comprising a central terminal and a plurality of subscriber terminals, wherein the central terminal comprises a transmission controller in accordance with the present invention, and at least one of the subscriber terminal comprises a reception controller in accordance with the present invention.
Viewed from a fourth aspect, the present invention provides a method of processing data items to be transmitted over a wireless link connecting a central terminal and a subscriber terminal of a wireless telecommunications system, a single frequency channel being employed for transmitting data items pertaining to a plurality of wireless links, the method comprising the steps of: (a) providing an orthogonal code from a set of xe2x80x98mxe2x80x99 orthogonal codes used to create xe2x80x98mxe2x80x99 orthogonal channels within the single frequency channel; (b) combining a data item to be transmitted on the single frequency channel with said orthogonal code, the orthogonal code determining the orthogonal channel over which the data item is transmitted, whereby data items pertaining to different wireless links may be transmitted simultaneously within different orthogonal channels of said single frequency channel; and (c) applying time division multiplexing (TDM) techniques to the data item in order to insert the data item within a time slot of the orthogonal channel, whereby a plurality of data items relating to different wireless links may be transmitted within the same orthogonal channel during a predetermined frame period.
Viewed from a fifth aspect, the present invention provides a method of processing data items received over a wireless link connecting a central terminal and a subscriber terminal of a wireless telecommunications system, a single frequency channel being employed for transmitting data items pertaining to a plurality of wireless links, and xe2x80x98mxe2x80x99 orthogonal channels being provided within the single frequency channel, the method comprising the steps of: (a) providing an orthogonal code from a set of xe2x80x98mxe2x80x99 orthogonal codes used to create said xe2x80x98mxe2x80x99 orthogonal channels within the single frequency channel; (b) applying, to signals received on the single frequency channel, the orthogonal code in order to isolate data items transmitted within the corresponding orthogonal channel; and (c) extracting a data item from a predetermined time slot within said orthogonal channel, a plurality of data items relating to different wireless links being transmitted within the same orthogonal channel during a predetermined frame period.
By using TDM techniques in addition to the known set of orthogonal codes, it is possible for selected orthogonal channels to be subdivided in the time dimension, thereby making it possible to support more wireless links on one frequency channel.